1. Field of the Invention
The present invention relates to a diffraction optical element or a transfer mold for such an optical element having a structure in which a orating shape adjusting layer is deposited on a grating surface constituted by a large number of grooves. Particularly, it relates to a diffraction optical element in Which an optical shape is changed so that a desired optical characteristic is exhibited by disposing a grating shape adjusting layer.
The invention can be also applied to a transfer mold for molding a diffraction optical element. A grating shape or the like of a molded product (a diffraction optical element or a replica molding tool) can be adjusted finely by forming a groove shape adjusting layer or by performing directional etching.
The diffraction optical element of the invention is particularly useful for a demultiplexing device or the like in wavelength division multiplexing optical communication.
2. Related Art
As known commonly, the diffraction optical element is an optical component using diffraction of light. As a typical example of the diffraction optical element, a large number of parallel grooves are formed on a surface at regular intervals to thereby form a diffraction optical element. For example, this type diffraction optical element is used as a demultiplexing device for taking out light having an optional wavelength from wavelength division multiplexed light in wavelength division multiplexing optical communication. There are various methods for producing such a diffraction optical element. A transfer molding method is one of the methods This is a method for producing a diffraction optical element by transferring a periodic groove structure of a transfer mold (original mold) Or a replica molding tool may be produced by transfer of a transfer mold (original mold) in advance so that a diffraction optical element is produced by retransfer of a periodic groove structure of the replica molding tool. In the case of a reflection type diffraction optical element, a reflecting film is formed on a grating surface of a molded product.
The periodic groove structure of a surface of the original mold is formed by an optional method such as a method of performing cutting by a ruling engine, a method of exposing a photo-curable resin applied on a substrate to coherent light and then dry-etching the substrate together with the resulting patterned resin, a method of dry-etching a glass substrate or a method of wet-etching a silicon substrate. For example, the density of the grooves formed is in a range of from several hundreds to a thousand and several hundreds per 1 mm. In the diffraction optical element produced by the transfer molding method, there is a certain limit to the shape of each groove because of a physical restriction on a producing method that mold release must be made. In addition, the depth of each groove is limited according to the shape of each groove but it is generally said that the limit in the depth of each groove is 400 nm to 600 nm.
The transfer molding method has an advantage in that it is excellent in mass production. On the other hand, when there is a shape failure found in the original mold after transfer molding, It is necessary to make the original mold again newly. There is a problem that a long time is required for making the original mold. The reason is that it is very difficult to correct the groove shape by an additional process such as cutting because the shape Lust be changed delicately.
Also in the case where a diffraction optical element having a desired optical characteristic slightly different from that of an existing diffraction optical element needs to be produced, it is necessary to make the original mold newly in accordance with the required optical characteristic. This is also because there has been no technique for performing an additional process on the basis of the original mold.